Solid state photonic devices are a class of devices in which the quantum of light, the photon, plays a role. They function by utilizing the electro-optical and/or opto-electronic effects in solid state materials. Because the interband optical transition (in absorption and/or in emission) is involved in photonic phenomena and because photon energies from near infrared (IR) to near ultraviolet (UV) are of interest, the relevant materials are semiconductors with band gaps in the range from 1 to 3 eV. Typical inorganic semiconductors used for photonic devices are Si, Ge, GaAs, GaP, GaN and SiC etc. Photonic devices are often classified into three categories: light sources (light emitting diodes, lasers, diode lasers etc.), photodetectors (photoconductors, photodiodes etc.) and energy conversion devices (photovoltaic cells) S. M. Sze, Physics of Semiconductor Devices (Wiley, New York, 1981)!. All three are important. Because photonic devices are utilized in a wide range of applications, they continue to provide a focus for research laboratories all over the world.
Conjugated polymers are a novel class of semiconductors which combine the optical and electronic properties of semiconductors with the processing advantages and mechanical properties of polymers. Semiconducting polymers typically have band gaps in the range from 1 to 3 eV. The molecular structures of a few important examples of semiconducting polymers are shown in FIG. 1. Because of the sp.sup.2 P.sub.z bonding of these planar conjugated macromolecules, each carbon is covalently bonded to three nearest neighbors (two carbons and a hydrogen); and there is formally one unpaired electron per carbon. Thus, the electronic structure (semiconductor or metal) depends on the number of atoms per repeat unit. For example the repeat unit of poly(paraphenylene vinylene), PPV, contains eight carbons; PPV is a semiconductor in which the fundamental p.sub.z -band is split into eight sub-bands. The energy gap of the semiconductor, the .pi.-.pi.* gap, is the energy between the highest occupied molecular orbital and the lowest unoccupied molecular orbital.
When functionalized with flexible side chains for example, poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene), MEH-PPV; see FIG. 1!, conjugated polymer materials become soluble in common organic solvents and can be processed from solution at room temperature into uniform, large area, optical quality thin films D. Braun and A. J. Heeger, Appl. Phys. Lett. 58, 1982 (1991)!. Because of the large elongation to break which is a characteristic feature of polymers, such films are flexible and easily fabricated into desired shapes that are useful in novel devices G. Gustafsson, Y. Cao, G. M. Treacy, F. Klavetter, N. Colaneri, and A. J. Heeger, Nature 357, 477 (1992).!
The goal of using such semiconducting polymers in "plastic" photonic devices is rapidly becoming reality. High performance photonic devices fabricated from conjugated polymers have been demonstrated, including light-emitting diodes J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns and A. B. Holmes, Nature 347, 539 (1990);D. Braun and A. J. Heeger, Appl. Phys. Lett. 58, 1982 (1991)!, light-emitting electrochemical cells Q. Pei, G. Yu, C. Zhang, Y. Yang, and A. J. Heeger, Science 269, 1086-1088 (1995)!, polymer grid triodes (a new architecture for plastic transistors) Y. Yang and A. J. Heeger, Nature 372, 344 (1994)!, polymer field-effect transistors F. Garnier, R. Hajlaoui, A. Yassar, P. Srivastava, Science 265 1684 (1994);A. Dodabalapur, L. Torsi and H. E. Katz, Science 268, 270 (1995)!, photovoltaic cells, and photodetectors G. Yu, J. Gao, J. C. Hummelen, F. Wudl and A. J. Heeger, Science 270, 1789 (1995)!, and optocouplers G. Yu, K. Pakbaz, C. Zhang and A. J. Heeger, J. Electron. Materials 23, 925 (1994).!; i.e. nearly all the categories which characterize the field of photonic devices. In many cases, such polymer-based devices have reached performance levels comparable to or even better than their inorganic counterparts.
Notably missing from this list, however, is the category representing solid state lasers fabricated from semiconducting polymers. The achievement of spectrally narrow solid state polymer lasers, either optically pumped or pumped by carrier injection, (i.e. laser diodes) is an important goal for polymer optoelectronic devices.
Laser emission has been observed from MEH-PPV in dilute solution in an appropriate solvent, in direct analogy with a conventional dye lasers D. Moses, Appl. Phys. Lett. 60, 3215 (1992); U.S. Pat. No. 5,237,582!. In this application, the diluted and dissolved luminescent polymer serves as the laser dye.
Solid state lasers require pumping a photoluminescent, solid material with suitable gain, said material contained within a resonant structure. The pumping can be done either optically (optically pumped lasers) or through carrier injection (carrier injection lasers). The photoluminescent material has gain if the stimulated emission is strong enough to exceed any losses from, for example, absorption from the ground state to an excited state, photoinduced absorption, or scattering. If the luminescent medium exhibits gain, Light Amplification by Stimulated Emission of Radiation (LASER) can be achieved if the optical path length in the medium exceeds the gain length. (The gain length, L.sub.g, of a laser material is defined as the distance required for the amplitude to increase by e.sup.+1.)
Many conjugated polymers exhibit relatively high photoluminescence (PL) efficiencies with emission that is shifted sufficiently far from the absorption edge that self-absorption is minimal. In such a case, stimulated emission, essential to the development of lasers, might be expected throughout the lifetime of the excited state. Ultrafast spectroscopic studies of poly(phenylenevinylenes) (PPVs) have revealed that stimulated emission is readily observed in solutions and dilute blend films M. Yan, L. J. Rothberg, E. W. Kwock and T. M. Miller, Phys. Rev. Lett. 75, 1992 (1995)J. M. Leng et al., Phys. Rev. Lett. 72, 156 (1994); J. W. Blatchford, Phys. Rev. Lett. 76, 1513 (1996); M. Yan, L. J. Rothberg, B. R. Hsieh and R. R. Alfano, Phys. Rev. B 49, 9419 (1994); M. Yan, Phys. Rev. Lett. 72, 1104 (1994); R. Kersting, Phys. Rev. Lett. 70, 3820 (1993); and poly(paraphenylenes) (PPPs) W. Graupner, Phys. Rev. Lett. 76, 847 (1996); W. Graupner et al., Chem. Phys. Lett. 246, 95 (1995); T. Pauck et al., Chem. Phys. Lett. 244, 171 (1995)!.
In neat solid films, however, stimulated emission typically either has not been observed or has been observed to decay within at most a few picoseconds. This absence of stimulated emission results from strong photoinduced absorption (PA) which overwhelms the stimulated emission in neat films but not when the polymer chains are isolated in solution or in dilute blends. The absence of observable stimulated emission implies that the excited medium does not exhibit gain. Without gain, polymer solid state lasers are not possible. On the other hand, since self-absorption of the luminescence by transitions from the ground state is not important, we have postulated that semiconducting luminescent polymers with gain can be achieved if photoinduced absorption which overlaps the emission spectrum is eliminated.
For semiconducting luminescent polymers the pump transition and the emission derive from the same electronic transition A. J. Heeger, S. Kivelson, J. R. Schrieffer, and W.-P. Su, Rev. Mod. Phys. 60, 781 (1988)!. The spectral Stokes shift arises from a combination of structural relaxation and disorder. In this case, general radiation theory indicates that the transition probability for absorption will be approximately equal to the transition probability for stimulated emission A. Yariv, Quantum Electronics, 3rd edn, (Wiley, New York, 1989)!. Therefore, L.sub.g .apprxeq.L.sub..alpha. /fractional population in excited state! where L.sub..alpha. .apprxeq.0.1-1 .mu.m or even less. Thus, under conditions of population inversion when the fractional population of the excited state approaches unity, the gain length in semiconducting polymers is in the micron or even sub-micron regime.
Materials with gain lengths in the micron and sub-micron regime are rare and very special; such high gain materials are the enabling feature of thin film solid state lasers, either optically pumped or pumped via carrier injection.
In semiconducting, luminescent polymers, the emission is at longer wavelengths than the onset of significant absorption (the Stokes shift). Because of the spectral Stokes shift between the absorption and the emission, there is minimal self-absorption of the emitted radiation H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Bums and A. B. Holmes, Nature 347, 539 (1990); D. Braun and A. J. Heeger, Appl. Phys. Lett. 58 1982 (1991)!. Thus, in semiconducting luminescent polymers, self-absorption need not make the materials lossy. Moreover, since the absorption and emission are spectrally separated, pumping the excited state via the .pi. to .pi.* transition does not stimulate emission. Thus, by pumping the .pi.-.pi.* transition, one can achieve an inverted population.
Because of the large joint density of states associated with the direct .pi. to .pi.* (interband) transition of these quasi-one-dimensional, semiconducting polymers, the absorption coefficient (.alpha.) is large, typically .alpha.=10.sup.5 cm.sup.-1 or greater A. J. Heeger, S. Kivelson, J. R. Schrieffer, and W.-P. Su, Rev. Mod. Phys. 60, 781 (1988)!.
Gain narrowing and amplified spontaneous emission (as needed for laser emission) in neat solid films of semiconducting luminescent polymers, neither diluted nor blended, has not been previously demonstrated. More specifically, gain narrowing and amplified spontaneous emission in thin solid films of semiconducting luminescent polymers, neither diluted nor blended, with thickness less than 10 .mu.m have not been previously observed.